1. Field of the Invention
The invention relates to a white light-emitting device configured by using a combination of a blue light-emitting element and phosphors.
2. Description of the Related Art
A white light-emitting device is known in which a light-emitting element such as blue LED chip is sealed with a sealing resin containing a yellow phosphor such as YAG phosphor (Y3Al5O12:Ce).
On the other hand, a white light-emitting device using red and green phosphors in combination with a blue light-emitting element has become popular on the ground that a color gamut thereof is wider than that of the white light-emitting device using the yellow phosphor such as YAG phosphor. Particularly, a combination of a KSF phosphor (K2SiF6:Mn) as a red phosphor with a β-SiAlON phosphor ((Si,Al)6(O,N)8:Eu) as a green phosphor has drawn attention since both have narrow emission spectra.
In the white light-emitting device using the KSF phosphor and β-SiAlON phosphor, color purity can be improved due to the narrow emission spectra. Phosphors having narrow emission spectra, however, have such properties that afterglow time is long and response speed is slow.
In addition, since light conversion efficiency of β-SiAlON phosphor and KSF phosphor is lower than YAG phosphor, etc., it is necessary to add a larger amount of β-SiAlON phosphor and KSF phosphor to the sealing resin than when using YAG phosphor, etc., which results in a problem that the sealing resin has a higher phosphor concentration and viscosity of the sealing resin is thereby increased. Such an increase in viscosity of the sealing resin causes stringiness, difficult uniform dispersion of phosphor, unstable liquid level and difficult defoaming during a process of sealing the light-emitting element, leading to a decrease in productivity and an increase in the manufacturing cost.
Against this background, a white light-emitting device is known which offers high color purity and high responsivity by using e.g., a sealing resin containing 13.7 wt % of CASN phosphor (CaAlSiN3:Eu) and 75.7 wt % of green phosphor with respect to 100 wt % of KSF phosphor (see e.g., WO 2015/068513).
Also, a white light-emitting device is known in which a Mn4+-activated fluoride complex phosphor and a Eu2+-activated alkaline earth silicon nitride phosphor are contained in a sealing resin such that the amount of the Eu2+-activated alkaline earth silicon nitride phosphor is not less than 0.5 wt % and not more than 14.0 wt % of the combined amount of all phosphors, and which offers appropriate viscosity of the sealing resin by reducing the amount of the Eu2+-activated alkaline earth silicon nitride phosphor as the KSF phosphor (see e.g., WO 2012/050199).